A shallow gene pool

Today's High-Yielding corn hybrids and soybean varieties didn't just emerge by accident. Significant investments in research, along with the tireless effort of plant breeders, have produced the new hybrids and varieties that continue to break through the yield ceiling.

But as plant breeders bring growers elite products with tremendous genetic potential, there is a concern that reduced genetic diversity in germplasm may slow yield gain, and perhaps open up the potential for widespread disease problems.

“There's been a lot written about the loss of genetic diversity in germplasm,” says Jode Edwards, crop geneticist with the USDA Agricultural Research Service (ARS). “What is generating some of the concern is that the genetic diversity within the elite germplasm pool is somewhat narrow.”

Of the approximately 250 to 300 races of maize in the world, virtually all the current corn hybrids came from Corn Belt Dent, a hybrid that traces back to crosses of Northern Flint with Southern Dent races in the early 1800s. More than 1,000 open pollinated hybrids were developed and grown during America's westward expansion. The present U.S. corn germplasm base is comprised mainly of derivatives of popular inbred lines such as B73, I205, Mo17, OH7, OH43 and A632. A narrow germplasm base limits the materials scientists can use to bring new traits into the pipeline.

The role of germplasm genetic diversity is complex. “The idea behind having diverse germplasm is to have a buffer in case of disease or insect problems,” says William Tracy, professor of agronomy at the University of Wisconsin — Madison. “But adding to the complexity of the issue is that we really don't know what the next big disease or insect problem might be.”

The issue of genetic diversity first became apparent in 1970, when the southern corn leaf blight hit. Tropical storms moving up the Mississippi Valley brought disease spores, rain and humidity to the Corn Belt, where 90% of the corn hybrids were susceptible because they shared the same plant cell cytoplasm (which is similar to the white of an egg). The disease reduced the U.S. crop by about 15%, but in certain areas of the country the losses were much higher.

“The southern corn leaf blight was an unusual event, in that the same cytoplasm, used to ease seed production, was susceptible to southern corn leaf blight and was used over many acres,” says Robert Nielsen, professor of agronomy at Purdue University. “We don't have that vulnerability in seed production today.”

Maintaining genetic diversity in seed germplasm is easier said than done. “No farmer is going to plant a low-yielding hybrid just to get genetic diversity,” says John Dudley, emeritus professor of agronomy at the University of Illinois. “Today's seed companies want to work with the best germplasm to get the best-yielding products. Lower-yielding products don't sell. No one has top sales on second-best hybrids.”

The idea of working with a narrower pool of germplasm is as old as plant breeding itself. Cross a good product with a good product, and theoretically you get a better product. That's the practical theory behind plant breeding.

“The question of whether we're running out of genetic variability in the pool that we're working with now is critical,” Dudley says. “However, if you look at the data, the patented lines and the amount of variability, it's hard to say how much variability is enough. Looking at the yield curve, soybean and corn yields continue to go up and the pace doesn't seem to be slacking, suggesting we have not run out of variability.”

The advent of biotechnology has helped boost yields significantly. But scientists say that biotech traits have played a larger role in helping to secure the higher yields locked in the seed germplasm.

“When it comes down to it, the biotech traits are great, but they do little good if there's not a strong genetic engine in the seed,” says Forrest Troyer, adjunct professor of agronomy at the University of Illinois. Genetic principles continue to be important. “Hybrid vigor is caused by genetic diversity. Hybrid corn necessarily has more genetic diversity than other non-hybrid crops,” Troyer says.

“Yield is a combination of so many individual pieces,” Edwards says. “Some come from the germplasm itself — the angle of the leaves, the ability to get silks out in dry weather. And some come from biotech traits — better weed control and insect resistance. You have to have the entire package to get yield. Break any piece of the chain and yield suffers.”

Keeping the library open

The race to higher yields may focus only on segments of the germplasm pool, but scientists continue to work at ensuring that the entire library of corn and soybean germplasm continues to remain available. That's because there's value in the code that these non-elite seeds hold.

Projects such as the ARS's Germplasm Enhancement of Maize Project (GEM) works to increase the diversity of U.S. maize germplasm used by producers, end-users and consumers.

The main objectives of the unique GEM project, based in Ames, IA, and Raleigh, NC, are to manage a multisite cooperative breeding and testing network of public and private investigator trials, develop genetically enhanced populations and inbred lines from GEM germplasm, and evaluate genotypes for yield, agronomic traits and value-added traits, according to geneticist/coordinator Michael Blanco. The ARS's Maize Genetics Stocks Collection, another valuable resource, is housed in Illinois.

GEM leverages the collection of largely unadapted, diverse maize races maintained by the National Plant Germplasm System (NPGS) and from many other sources.

The NPGS is a joint cooperative effort between ARS and public and private entities to preserve and use inherent genetic diversity of crop plants and their relatives. The system acquires, preserves, evaluates and documents crop germplasm and distributes plant genetic resources to researchers all over the world to help them accomplish their objectives.

Soybean genes

According to the National Soybean Research Laboratory, genetic variation in commercial soybean varieties grown in the United States is limited, with only three ancestors providing 50% of all the genes, which may limit the potential for genetic improvement of soybeans.

The USDA Soybean Germplasm Collection housed at the University of Illinois is a repository of the available natural variation that exists for soybeans. And although transgenic traits have become more widespread, “much of what has been commercialized for transgenics are genetically simple characteristics controlled by single genes,” says Randall Nelson, professor of plant genetics, and research leader with the ARS. “Much of what we depend on for the economic value of the crop is genetically very complex.”

Germplasm is still the basic tool plant breeders have to make important changes in new varieties. Transgenics may be grabbing the headlines, but basic plant breeding is still critical.

The USDA soybean collection at the University of Illinois is heavily used. “In the past decade, we have distributed almost 300,000 seed samples of soybeans and relatives of soybeans to more than 1,500 users,” Nelson says. Users include more that 100 universities in 47 states, as well as 150 different commercial companies. “There is a large scope of people who use this material,” he says.

Changing landscape

“In the course of my career, soybean breeding programs have shifted significantly from the public sector to the private sector,” Nelson says. “Now in major soybean production states, almost all of the commercial varieties are from private industry.

“Entries in our germplasm collection may have some very useful traits, but do poorly agronomically,” Nelson continues. “Our work is to identify these traits and incorporate them into improved experimental lines. These lines do not have all of the qualities needed for a commercial variety but can be more easily used by private breeders as parents to develop improved varieties.”

Now the roles of the USDA and universities in crop improvement are to research plant breeding and to protect genetic diversity.

Maintaining sources of genetically diverse seed germplasm is important — not only to preserve current hybrids and varieties, but to protect the crops that will be grown to feed future generations.